In the most common knee arthroplasty surgeries, the proximal end of the tibia and distal end of the femur are removed and replaced with metal and/or polymer prosthetics.
Long bones, including both the tibia and femur, include cortical bone and trabecular or cancellous bone. Cancellous bone is spongelike as it typically has form resembling a mesh, and is found primarily near ends of the bone, where its open-celled mesh supports cortical bone of the bearing surface at each joint. The mesh of cancellous bone is not empty, it is typically filled with soft tissue such as fat and marrow. Cortical bone is denser and more solid than cancellous bone, and not only forms a surface over cancellous bone but typically forms much of the long shaft portion of each long bone, as well as tendon and ligament attachment points and the bearing surfaces (articular surfaces) of each joint; typically the cortical bone bearing surfaces of the bones at each joint are separated by articular cartilage padding and lubricated with synovial fluid.
During arthroplasty of a joint involving long bones, some or all articular cartilage remaining in the joint is removed—indeed many arthroplasties are performed because of joint pain arising because the articular cartilage has been destroyed by injury or arthritis. Bone at the bearing surface of one or more bones of the joint is then trimmed back to make room for, and to fit onto, an implant that will functionally replace the bearing surface of that one or more bones. In the process of trimming back the bone, much cortical bone of the bearing surface, and some adjacent cortical bone, is removed, leaving residual bone, much residual bone at trimmed surfaces is newly exposed cancellous bone. Once bone trimming is completed, bone cement is used to attach the implant to the remaining bone, including to cancellous bone, of the long bone. The most common bone cement used in 2015 is based on polymethyl methacrylate (PMMA) with radiopaque filler and other additives.
In a total knee-replacement arthroplasty, implants cemented to that part of each bone remaining after bones are trimmed replace the articular surfaces of both femur and tibia, and in many such surgeries the articular surface of the patella.
A typical prior-art total knee replacement arthroplasty may use implants as illustrated schematically in FIG. 1A as assembled into a patient, and in FIG. 1B as an exploded diagram showing each individual implant. A first implant 102, or femoral component, is formed with a socket 104 that attaches to residual bone of femur 105. A second implant 106, or tibial component, is formed with a stabilizing protrusion 108 that is configured to extend distally from the joint into a slot trimmed into residual tibia 110. A plastic insert 112 disposed between the first 102 and second 104 implants serves to pad and lubricate the joint and keep the first and second implants from abrading each other. A third implant 114, or patellar component, may in some cases be affixed to the underside of the patella (not shown), where it slides on first implant 102 as the knee bends.